1. Field of the Invention
The present invention relates generally to ambient air vaporizers and heaters and more particularly to a surge dampening device for unidirectional flow vaporizers and heaters that use cryogenic fluids operating below fluid critical pressures.
2. Brief Description of the Prior Art
Atmospheric vaporizers which are well known in the art are used in many industries to vaporize relatively large quantities of a cryogenic liquid which is needed in the gaseous form for various manufacturing processes and other operations. A typical example for the use of atmospheric vaporizers/heat exchangers is the vaporization of liquid oxygen for use in industrial welding operations. Basically, an atmospheric vaporizer is a heat exchanger which utilizes ambient heat to vaporize a very low boiling (cryogenic) liquid.
State-of-the-art atmospheric vaporizers/heat exchangers include a plurality of heat exchange elements which are finned tubes made of good heat conducting materials such as aluminum. The finned tubes are mechanically assembled to one another and to a substantially rigid frame. Flow of the cryogenic fluid through the tubes is generally in a serial fashion; that is, from one tube to another to maintain the height of the vaporizer/heat exchanger within reasonable limits, for example, of the order of 6 to 25 or more feet. The relatively large surface of each fin facilitates efficient heat exchange with the environment; in other words, the fins promote relatively efficient absorption of heat required for vaporization and heating of the cryogenic liquid.
As an example of a prior-art serial atmospheric vaporizer/heat exchanger one may refer to U.S. Pat. No. 4,479,359 which includes specific dimensions for the external and internal fins of its elongated finned tubes.
One may also refer to the ambient air vaporizer and heater for cryogenic fluids disclosed in U.S. Pat. No. 5,251,452, ("'452 patent") assigned to the assignee of this application, wherein the present surge dampening device is readily usable therewith as well as with other like vaporizer/heater units.
It is well known in the art that dynamic instability of fluid flow, that is surging, in cryogenic vaporizers often occurs and presents various operational problems. Dynamic instability in flow (surging) in cryogenic vaporizers is a complex function of a number of variables, including vaporizer flow channel geometry, mode of heat transfer, heat flux and flux distribution, degree of subcooling of the inlet liquid, and operating pressures, as well as flow rate and fluid properties. The instability for the type of ambient air vaporizer described herein is caused primarily by a combination of density wave oscillations and pressure wave oscillations. These oscillations are induced by the forced convection film boiling characteristics of subcooled cryogenic fluids, and are considered to be driven by thermal responses of the vapor film to these flow disturbances, which in turn alters heat transfer behavior and pressure drop and further promotes flow disturbances.
Prior art for dampening surging in cryogenic vaporizers typically includes the use of individual orifices in the flowing process stream at the inlet of each parallel flow circuit (each heat transfer element) which constitutes the vaporizer, or a single orifice at the liquid inlet to the vaporizer. However single or multiple inlet orifices alone do not necessarily provide the proper dampening for the operating conditions desired. This is because quite often the amount of orifice restriction in the flowing process stream required to dampen surges to reasonable levels for a particular vaporizer application very often restricts flow to an amount below the required rated flow of the vaporizers, and can also result in a pressure drop that causes flashing of the liquid. The liquid flashing, in turn, creates a two-phase flow condition at the inlet to each parallel flow circuit in the vaporizer, thereby resulting in mal-distribution and reduced capacity of the vaporizer.
Additionally, a vertical surge leg is often provided in the liquid supply line just upstream of the vaporizer in combination with an orifice upstream of the surge leg. The surge leg is uninsulated and allows liquid to enter. Since the leg is uninsulated, heat leak results in a vapor phase at the top of the closed end leg which acts as a spring. Such a device in conjunction with the upstream orifice (both external to the vaporizer) reduces surging by trapping pluses traveling back to the source of liquid supply. Other schemes external to the vaporizer are also used which rely on large gas volumes downstream of the vaporizer in the gas phase are also used.
Further, where liquid supply pressure is high and large pressure drops are permissible, "twisted-tape" type turbulators may be inserted into the flow circuits to enhance heat transfer and provide dampening. However, for relatively low operating pressures, where low pressure drop criteria applies, turbulators do not provide effective dampening.